Natural and Enhanced Remediation Systems - Chapter 7 ppt

©2001 CRC Press LLCCHAPTER 7 Engineered Vegetative LandÞll Covers CONTENTS 7.1 Historical Perspective on Landfill Practices7.2 The Role of Caps in the Containment of Wastes7.3 Convention

Suthersan, Suthan S “Engineered Vegetative Landfill Covers”

Natural and Enhanced Remediation Systems

Edited by Suthan S SuthersanBoca Raton: CRC Press LLC, 2001

©2001 CRC Press LLC

CHAPTER 7 Engineered Vegetative LandÞll Covers

CONTENTS

7.1 Historical Perspective on Landfill Practices7.2 The Role of Caps in the Containment of Wastes7.3 Conventional Landfill Covers

7.4 Landfill Dynamics7.5 Alternative Landfill Cover Technology 7.6 Phyto-Cover Technology

7.6.1 Benefits of Phyto-Covers over Traditional RCRA Caps7.6.2 Enhancing In Situ Biodegradation

7.6.3 Gas Permeability7.6.4 Ecological and Aesthetic Advantages7.6.5 Maintenance, Economic, and Public Safety Advantages7.7 Phyto-Cover Design

7.7.1 Vegetative Cover Soils7.7.2 Nonsoil Amendment7.7.3 Plants and Trees 7.8 Cover System Performance7.8.1 Hydrologic Water Balance7.8.2 Precipitation

7.8.3 Runoff7.8.4 Potential Evapotranspiration — Measured Data7.8.5 Potential Evapotranspiration — Empirical Data7.8.6 Effective Evapotranspiration

7.8.7 Water Balance Model7.9 Example Application7.10 Summary of Phyto-Cover Water Balance7.11 General Phyto-Cover Maintenance Activities 7.11.1 Site Inspections

7.11.2 Soil Moisture Monitoring

7.11.2.1 Drainage Measurement7.11.3 General Irrigation Guidelines 7.11.4 Tree Evaluation

7.11.4.1 Stem 7.11.4.2 Leaves 7.11.5 Agronomic Chemistry Sampling7.11.6 Safety and Preventative Maintenance7.11.7 Repairs and Maintenance

7.12 Operation and Maintenance (O&M) Schedule7.12.1 Year 1 — Establishment

7.12.2 Years 2 and 3 — Active Maintenance7.12.3 Year 4 — Passive Maintenance 7.13 Specific Operational Issues

7.13.1 Irrigation System Requirements 7.13.2 Tree Replacement

References

Maintaining and enhancing the closed landfill as a bioreactor requires cation of design and operational criteria normally associated with traditional landfill closure…

modifi-7.1 HISTORICAL PERSPECTIVE ON LANDFILL PRACTICES

The practice of using shallow earth excavations, or landfills, for disposal of liquidand solid waste has a very long history Landfill practices basically followed thedesign philosophy of “out of sight, out of mind” in that a pit or trench was excavatedinto the ground, waste was placed into the excavation, and, when it was full, theexcavation was covered with soil and abandoned If thought was ever given to thematter, it was likely assumed that the soil surrounding the waste effectively preventedcontaminant migration from the burial zone

It was not until 1976, with the passage of the Resource Conservation andRecovery Act (RCRA), and 1980, with the passage of Comprehensive EnvironmentalResponse, Compensation, and Liability Act (CERCLA) that federal and state regu-lations mandated much improved methods for disposal of waste in landfills Todaythere are a plethora of federal and state regulations controlling all aspects of landfilldisposal of municipal, radioactive, and hazardous waste The problem in the U.S.,however, is that hundreds of thousands of landfills were operated and then decom-missioned prior to the requirements of current regulations Many of these old landfillsnow come under the closure requirements of RCRA or CERCLA, depending on theagreements between the responsible parties In 1989, U.S Environmental ProtectionAgency (USEPA) stated that there are 226,000 sanitary landfills in the U.S requiringevaluation for potential risks to human and environmental receptors.1

Regardless of the corrective action imposed on these old sites, almost all of themwill require installation of a new cover as a final step in the closure process The

design of most landfill covers in the U.S has been based on criteria developed byEPA for use in closing either RCRA subtitle C (hazardous waste) or subtitle D(municipal solid waste) landfills Two major themes emerge in reviewing recentwork in landfill cover design:2 1) there has been an overemphasis on regulatorycompliance, thus inhibiting innovative and creative design that looks at the entirelandfill system as a holistic biogeochemical environment, and 2) there are fewpublished data on field performance of constructed cover systems and their impacts

on the biogeochemistry of the groundwater within the footprint of the landfill

7.2 THE ROLE OF CAPS IN THE CONTAINMENT OF WASTES

Because of the expense and risk associated with treating or removing largevolumes of landfill wastes, remediation usually relies upon containment, whichrequires the construction of a suitable cover Both regulators and the public usuallyaccept covers as part of the presumptive remedy for final landfill remediation;therefore, covers are likely to be included in the optimal remedial actions for closure

of most landfills

The intent of landfill remediation is to protect the public health and the ment In keeping with this intent, a modern philosophy has evolved requiring con-taminants in the waste to be isolated from receptors and contained within the landfill

environ-As a result, landfills have become warehouses in which wastes are stored for anindefinite time, possibly centuries

There are fundamental scientific and technical reasons for placing a cover onlandfill sites Although regulations are often the most apparent influence governingthe selection and design of landfill covers today, these regulations were draftedbecause of specific environmental concerns and were based upon scientific andtechnical understandings The three primary requirements for landfill covers are to:

• Minimize infiltration: water that percolates through the waste may dissolve contaminants and form leachate, which can pollute both soil and groundwater as

it travels from the site.

• Isolate wastes: a cover over the wastes prevents direct contact with potential receptors at the surface and prevents movement by wind or water.

• Control landfill gas: landfills may produce explosive or toxic gases, which, if allowed to accumulate or to escape without control, can be hazardous.

Landfills have been covered by barriers for years, usually built with little regardfor the monetary and environmental costs associated with constructing and main-taining them A typical landfill cover design consists of a sequence of layeredmaterials to control landfill gas infiltration and promote internal lateral drainage.The uppermost layer of a landfill cover consists of a vegetative soil layer to preventerosion, promote runoff, and insulate deeper layers from temperature changes Thelandfill cover is not a single element but a series of components functioningtogether.3

Landfill covers are designed to minimize infiltration of rainfall and melting snowinto the landfill in order to minimize postclosure leachate production This objective

is achieved by converting rainfall into surface runoff and infiltration into piration and lateral drainage without compromising cover integrity Secondary per-formance objectives of landfill cover design include the following:3 minimize post-closure maintenance; return the site to beneficial use as soon as possible; make thesite aesthetically acceptable to adjacent property owners; accommodate post-closuresettlement of the waste; address gas and vapor issues; provide stability againstslumping, cracking, and slope failure; provide resistance to disruption by animalsand plants; and comply with landfill closure regulations

evapotrans-The design features of a landfill cover are varied to affect changes in the overallwater balance within the landfill to meet primary landfill cover objectives The designadopted must take into account numerous other considerations, including costs, longterm maintenance implications, and construction risks The relatively large areasthat landfill covers protect, and the thickness and number of individual layers withinthem, make covers a cost-intensive component of landfill facility design

7.3 CONVENTIONAL LANDFILL COVERS

Nearly all conventional landfill covers in current use incorporate a barrier withinthe cover The “impermeable” barrier layer is intended to prevent water from movingdownward in response to the force of gravity In effect, these covers are designed

to oppose the forces of nature Barrier-type covers commonly include five layersabove the waste (Figure 7.1).1 The top layer consists of cover soil typically two feetthick and supports a grass cover that provides erosion control The barrier layerconsists of either a single low-permeability barrier or two or more barriers incombination The fourth layer is designed to remove landfill gases as they accumulateunderneath the barrier layer The bottom layer is a foundation layer of variablethickness and material; its purpose is to separate the waste from the cover and toestablish sufficient gradient to promote rapid and complete surface drainage fromthe finished cover

The barrier layer is the defining characteristic of conventional landfill covers Itmay be composed of compacted clay, a geomembrane, a clay blanket, or two ormore layers of materials in combination A compacted clay layer is frequentlyspecified to have a maximum saturated hydraulic conductivity (K) £ 1 ¥ 10–7 cm/sec

In contrast, both the drainage and gas collection layers are constructed to enhanceflow and commonly contain washed and selectively sieved sand, gravel, or speciallydesigned synthetic materials

The soil in the top layer of barrier-type covers is usually too thin or has inadequatewater holding capacity to store infiltrating precipitation during a large storm Thesecovers rely on barrier layers and rapid drainage through lateral drainage layers toprevent precipitation from reaching the waste Barrier-type covers must accommo-date specific site conditions, and supplemental components are sometimes added.For example, gravel may be added to the surface soil in desert regions to control

wind erosion, or a layer of cobbles may be used with the cover to discourage animalburrowing into the waste.

7.4 LANDFILL DYNAMICS

Landfills that contain a large amount of organic, putrescible materials (such asmunicipal solid waste) literally function as bioreactors Most “landfill bioreacters” ingeneral contain anaerobic and/or facultative microorganisms Landfill leachate is gen-erated as a result of the percolation of water or other liquids through the waste andalso due to the accumulation of moisture generated as a result of microbial degradation

of waste Leachate is a concentrated fluid containing a number of dissolved andsuspended materials, specifically, high concentrations of organic compounds (organicacids, hydrocarbons, etc.) and certain inorganic compounds (ammonia, sulfates, dis-solved metals, etc characteristic of the parent waste materials) from which it is derived

In addition, natural microbial activity in landfills also results in the generation of gasessuch as methane, carbon dioxide, ammonia, and hydrogen sulfide, a fraction of whichwill be dissolved in the leachate and may be introduced into the groundwater.Numerous landfill investigation studies4 have suggested that the stabilization ofwaste proceeds in sequential and distinct phases The rate and characteristics ofleachate produced and biogas generated from a landfill vary from one phase toanother and reflect the processes taking place inside the landfill These changes aredepicted in Figure 7.2

Waste Foundation Layer

Protective Cover Layer

Vegetative Layer 0'-6"

1'-18"

2'-30"

(min.)

Vegetation Topsoil Common Borrow Material Geocomposite

(Textile-Net-Textile) 40-mil LLDPE

The initial phase is associated with initial placement of waste and accumulation

of moisture within landfills Favorable biochemical conditions are created for thedecomposition of waste During the next phase, transformation from an aerobic toanaerobic environment occurs, as evidenced by the depletion of oxygen trappedwithin and introduced to landfill media and continuous consumption of nitrates andsulfates Subsequent phases involve the formation of organic acids and methane gas.During maturation phase, the final state of landfill stabilization, available organiccarbon and nutrients become limiting, and microbial activity shifts to very low levels

of activity Gas production dramatically drops and leachate strength remains constant

at much lower concentrations than in earlier phases

Biochemical decomposition of putrescible solid waste is shown below by anexample (Equation 7.1) Typical landfill gas composition during peak activity as abioreactor is: 60% methane, 40% carbon dioxide, 5–10% other gases, and 0.3–1.0%VOCs and non-monitored organic compounds Gas generation rates during peakactivity typically fall within the ranges of 5–15 ft3 per pound of refuse per year.9

(7.1)

Due to very high gas pressures generated at the source areas within the landfill(up to 4 atmospheres), migration of dissolved contaminants into the gaseous phasecould be a serious concern Contaminants transferred into the gas phase could bereadsorbed in the waste above the water table, or dissolve in the moisture, condense

in the waste zone, or migrate away from the landfill The potential for contaminantmigration from the dissolved phase into the landfill gas can be evaluated as shown

in Equation 7.2, and Figure 7.3

C H O H O6 10 5 2 Anaerobic 3CH4 3CO2

Under non-equilibrium conditions:

(7.2)

where

= transfer rate from gas to water

K = phase transfer coefficient

H = Henry’s Law Constant of VOC

Cg = gas phase concentration of VOC

Cw = water phase concentration of VOC

A = gas/liquid contact area

The progress toward final stabilization of any landfill and the organic waste in

it is subject to physical, chemical, and biological factors within the landfill ment, age and characteristics of the waste, operation and management controlsapplied, as well as site-specific external conditions

environ-Although barrier layers are sometimes referred to as “impermeable” layers, inpractice this is seldom true An extensive review of failures and failure mechanismsfor compacted soil covers in landfills was performed and emphasized that “…naturalphysical and biological processes can be expected to cause [clay] barriers to fail inthe long term.”5 Another study discussed a field test conducted in Germany in which,

dm

dt =K C( g-HCw)A

dm

dt

seven years after construction, percolation through the compacted clay was almost

200 mm/yr and increasing Geomembrane barriers are also prone to leak.6 Othershave traced most leaks in geomembranes to holes left by construction.7,8

A modification of the typical barrier cover is the subtitle D cover (Figure 7.4)that relies on compaction to create a layer of soil with reduced K value Usedprimarily for municipal landfills in dry regions, its use and components are specified

in subtitle D of RCRA (40 CFR, Part 258.60), hence the name From the surfacedownward, the cover includes an erosion control layer and a layer of compactedsoil A major advantage of the subtitle D cover is that its construction cost is lowerthan for an RCRA subtitle C cover Even though it has gained regulatory and publicacceptance, the subtitle D cover cannot ensure long-term protections against infil-tration of water into the waste, even in dry regions, because 1) the topsoil layer haslimited water holding capacity, 2) there is no drainage layer, 3) few roots can grow

in the barrier layer to remove water, and 4) soil freezing and root activity are likely

to increase the K value of the barrier soil layer over time

0.47 m 0.15 m

Waste

Soil Barrier

K ≤1 x 10-5 cm/sec Topsoil Precipitation

Foundation GravelRunoff

-7.5 ALTERNATIVE LANDFILL COVER TECHNOLOGY

Alternative covers to the RCRA subtitle C or D design include evapotranspiration(ET) covers and capillary barriers The ET cover uses no barrier or horizontaldrainage layers; it is designed to work with the forces of nature rather than attempting

to control them An ET cover in its simplest form is a vegetated soil cover with asufficiently deep soil profile so that infiltrated water is stored until removal byevaporative losses from the soil surface and by plant roots at depth in the profile Acapillary barrier also relies on water removal by ET, but is designed such that waterstorage near the surface is enhanced to promote the efficient removal of infiltratedwater by the ET process Optimization of material types and thicknesses for capillarybarriers is critical to their effective performance The use of sands or clays as thefine-soil component in the capillary barrier has proven to be less effective in storingwater than silt loams Capillary barriers can be thought of as enhanced ET covers

— alternative cover systems that work best in semi- and/or arid environments wherehigh ET rates and low precipitation make it possible to remove all infiltrated water

by ET However, even in arid environments there are situations where ET coversand capillary barriers can allow excessive percolation, particularly where the soilused in the cover design has insufficient storage capacity to accommodate wintersnow melt events

7.6 PHYTO-COVER TECHNOLOGY

The phyto-cover is the most popular application of the ET cover and is anengineered agronomic system that harnesses the natural transpiration process ofplants to limit percolation to the groundwater A phyto-cover relies on shallow- anddeep-rooted plants to create a thick root zone from which the plants can extractavailable moisture In effect, the plants serve as natural, solar-powered “pumps” towithdraw soil moisture and either convert it into biomass or evaporate it throughtheir leaves The withdrawal rate of the botanical pumps is limited by the availableenergy (sunlight), rate of growth, and available soil moisture; withdrawal virtuallyceases during winter dormancy Accordingly, the depth and composition of the rootzone must be sufficient to store accumulated water like a sponge and hold it untilthe plants remove it Properly designed, this “sponge and pump” water removalsystem (Figure 7.5) can limit water from percolating below the root zone and can

be equally protective of groundwater as a RCRA cap Thus, a phyto-cover serves

as a functional alternative to natural clay, geocomposite, or geosynthetic membranecap, yet offers several advantages over those technologies

The effectiveness of poplars in maintaining low soil moisture levels was firstdocumented by data collected from a phyto-cover application in Iowa.10 The phyto-cover consistently maintained soil moisture levels substantially below the soil’s fieldcapacity (i.e., the amount of water that soil can retain without allowing percolation)

of 40–45% Soil dryness was maintained by the trees’ prodigious water extractingability The capacity of certain trees such as hybrid poplar and willow trees to extractsoil moisture has been demonstrated by monitoring data from landfill at many sites

The poplars are employed at this site not as a cover, but to treat collected landfillleachate, which is applied to the poplars during the growing season The total amount

of water extracted from the soil in one growing season by these two- and old poplars was equivalent to about 62 inches of precipitation.10

three-year-One of the most important design considerations for a phyto-cover is choosingappropriate tree species and varieties Selected trees must be capable of achieving thedesired treatment objective and adapt to the irrigation water, soils, and climate of thesite Typically, achieving the highest possible rate of evapotranspiration is an importantgoal Critical site conditions for plant selection include soil chemistry, irrigation water

or groundwater chemistry, and adaptation to pests and diseases of the area Any factorthat compromises tree health and growth will reduce performance For example, hybridpoplar clones that include either trichocarpa or maximowiczii parents are quite sus-ceptible to Septoria canker if used in the U.S midwest.10

Especially for the commonly used Salicaceae, a number of different types ofplant materials may be used These include stem cuttings, whips or poles, and bareroot or potted material Use of larger or rooted plant material will result in morerapid establishment and reduced weed competition, but plant material and plantingcosts are much higher than for smaller material Whips and poles are commonlyused for deep planting applications Economics, especially planting costs, drive mostlarger installations (>5 ha) toward short stem cuttings

Certain varieties may result in a more valuable final wood product because ofstraighter stems or better paper processing properties Significant differences indamage from voles has been observed among hybrid poplar trees at phytoremediationsites Salt tolerance is a very important selection criterion, as differences betweenspecies and varieties can be significant Only limited data for Salicaceae are currentlyavailable to guide design, but a number of relevant research programs are ongoing.For an increasing number of sites, use of non-native species is unacceptable forlocal community groups and sometimes for regulators Use of native material willgenerally ensure resistance to local pests and disease, but may not afford the greatestefficiency

Once the tree system forms a complete canopy, spacing has little effect onevapotranspiration or nutrient requirements The impact of spacing on hydraulic andnutrient loading is primarily an early establishment phase concern Establishingdense initial plantings with the intention of thinning may provide small increases inearly capacity, but thinning operations are often neglected and the resulting maturetree stands are excessively dense Enough space must be left between tree rows toallow planned maintenance activities such as mowing or spraying.

The engineered phyto-cover system consists of densely planted, deep-rootedtrees and understory vegetation (perennial rye grass and clover) Photographs ofhybrid poplar tree stands of varying ages are shown in Figure 7.6 The water-holdingroot zone (“sponge”) includes the existing topsoil and fill at the site (includingintermediate and daily cover soil) supplemented with additional soil or soil amend-ments as dictated by design calculations A phyto-cover will provide a protective,living “skin” for a landfill that permanently heals the wound to the landscapeoriginally created by anthropogenic activities This skin can equal the percolation-blocking performance of a “rain coat” RCRA cap while being substantially morecost effective and providing additional benefits The final design of a phyto-coveroften includes provisions for monitoring soil moisture levels to ensure that perfor-mance criteria are met

Engineered phyto-cover systems have been applied to contain spilled icals and cover landfills, as well as buffers to remove nitrogen from industrial andmunicipal wastewater Sites where phyto-covers have been installed and recentresearch and demonstration sites for phyto-cover systems include the following:2,10-13

petrochem-• A 15-acre construction debris landfill in Beaverton, OR was covered with trees

in 1990 as an alternative to excavation of the fill, the installation of a liner, and then recovering with a geomembrane The phyto-cover is serving to protect groundwater cost-effectively The owner has continued to expand the cover as new areas are closed.

phyto-cover (courtesy of Licht, 1998).

Four-year Old Poplar Trees Two-year Old Stand of Poplars

• From 1992 to 1993, the Riverbend Landfill in McMinneville, OR planted a acre phyto-cover to manage landfill leachate water and soluble compounds All nutrient and water cycling results indicate the cap is achieving all regulatory requirements for ammonia treatment and ground protection.

17-• From 1993 to 1995, a 15-acre perimeter buffer was planted to reduce infiltration from upgradient runoff, grow a visual and sound barrier, and intercept downgra- dient leachate seepage Data collected at the site indicate the cap has been suc- cessful in stopping all leachate.

• At the Grundy county Landfill in Grundy Center, IA, a two-acre cap and perimeter buffer was planted from 1993 to 1994 to reduce leachate formation by installing

a phyto-cap over a completed subtitle D cap The cap also provides a visual and sound barrier, and intercepts downgradient leachate drainage.

• A three-acre poplar cap was planted in 1994 at the Bluestem 1 Landfill in Cedar Rapids, IA over a pre-subtitle D cap The cap serves to reduce leachate formation vertically and intercepts downgradient leachate drainage The data collected at this site have been used in writing specifications for soil and compost cover requirements and use of MSW waste as a planting media.

• A five-acre cap and perimeter boundary were planted in 1994 over a pre-subtitle

D cap at the Bluestem 2 Landfill in Marion, IA to reduce leachate formation Moisture management data from this cap have been used in subtitle D equivalence comparison between a soil/clay cover and the “sponge and pump” concept for deep rooted trees in four feet of rootable soil.

• In 1995, a ten-acre area was planted with poplar trees and a clover/grass understory over a subtitle D cell filled with MSW and industrial waste The Department of Environment Quality and governor’s office were interested in future phytoclosures for many funded pre-RCRA landfills in Virginia that have been abandoned and are creating potential environmental risk The trees are growing well and are being maintained by the owner A soil moisture measurement system using time domain reflectometry (TDR) is used to monitor the impact of tree roots on vadose zone water content A drip irrigation system using collected storm water can control the water stress during periods when moisture in the root zone has been exhausted.

• At a railroad RCRA site in Oneida, TN, a one-acre area impacted by coal and creosote from past manufacturing activities was covered with poplar trees and grass in 1997 The site soils were amended with compost and mineral fertilizer, then trenched in the root zone The trees and grass managed to accelerate biomass growth with resulting water uptake and in situ constituent removal The site groundwater is monitored by a university research grant to measure groundwater elevation and the containment of constituents The concept is similar to landfill capping where the phytosystem pumps water at high rates during the growing season and minimizes groundwater movement during the dormant season.

• The Woodburn WasteWater Treatment Plant in Woodburn, OR has been a onstration site since 1995; a full-scale installation took place in 1998 This site is the first full-scale tertiary treatment of secondary municipal wastewater effluent and is being designed for no leakage through the root zone in the summer months.

dem-• The Mid-Lakes Co-op site in Bonduel, WI used an aesthetically pleasing poplar cover over a spill plume to contain pollutant migration, make use of all available precipitation, protect public exposure, and remove constituents from the ground- water Closure requirements for this site included planting trees, monitoring the depth to groundwater, and monitoring groundwater quality over a three-year- period.

• In Staten Island, NY a phyto-cover consisting of poplars, willows, paulowia, and grasses is being used to prevent constituent migration and formation of leachate Enhancement of existing vegetation is expected to establish hydraulic control of groundwater by reducing water infiltration through the landfill materials.

• Evidence collected at a closed landfill in Elmore, OH indicates naturally occurring trees have created a treatment barrier for leachate seeps An evaluation of on-site box elder and osage orange trees yielded evidence of TCE uptake An evaluation

of the existing cover for supplemental enhancement for additional groundwater remediation and restoration was then conducted.

• A poplar tree phyto-cover was installed in 1996 at a landfill in Acme, NC The trees were planted in the most downgradient area of the landfill to stop leachate migration Groundwater constituent concentrations have dropped substantially in the area of the poplar trees but not in areas where trees were not planted.

• From 1992 to 1993, over 2000 poplar trees were planted at a site in Anderson,

SC to be used for processing waste from mining ore material The waste was used

to fill low areas over six acres of the site Data collected at the site indicate infilitration and leachate formation is being controlled.

• In 1991, a succession of trees (willow and black locust), legumes, and grasses were planted to dewater slurry waste at a site in Baton Rouge, LA The waste material was in a slurry state from a depth of 6 inches to 30 feet below ground surface The planted vegetation reduced the hydrated state of the waste and the occurrence of leachate through the impoundment.

• A process waste was placed as a slurry into an impoundment at a site in Texas City, TX Naturally occurring trees (osage, orange, and mulberry) and vegetation have reduced the hydrated state of the top ten feet of the waste Research on the site has found that dewatering and net water removal are directly correlated to the size of the trees.

• Ongoing research, funded by the USEPA Great Plain and Rocky Mountain ardous Substance Research Center involves planting trees at CERCLA sites to control erosion and leaching of zinc, arsenic, lead, and cadmium.

Haz-• A grass/soil cover system is one of five alternative covers being evaluated by Sandia National Labs in NM as part of an alternative landfill cover demonstration study Similar phyto-cover systems are being considered as potential demonstra- tion sites by USEPA ORD at sites in Tulsa, OK; Beatty, NV; and Hill Air Force Base in CA.

• A phyto-cover system has been proposed and designed at the 95% completion level for the F.E Warren Air Force Base Superfund Site in Cheyenne, WY This site is currently being considered as a technology demonstration candidate by USEPA-Region VIII.

• Pfitzer junipers have been used in a landfill cover field demonstration at Beltsville,

MD The juniper phyto-cover was installed over a clay layer to add to the “robust” cover development, but not as a replacement of the low-permeability layer The objective of the demonstration study was to document the influence of junipers

as water scavengers, yet maintain the water runoff performance of the permeability cap Compared to a reference soil, the “bioengineered” juniper cover reduced infiltration; it was demonstrated that the mature plant system improved the system’s resilience to failure.

low-• Research regarding the establishment of sufficient vegetation to provide adequate biomass growth with resultant evapotranspiration is being conducted at Idaho National Engineering Lab, Idaho Falls, ID This research focuses on four plant

species to deplete soil moisture, and the configuration of a capillary barrier and root zone to prevent deep percolation during wet periods The use of such phyto-covers has been demonstrated to be applicable to landfill sites in the semi-arid west.

• In Ljubljana, Slovenia, a ten-acre cover was planted with poplar trees in 1993–1994 with the primary goal of protecting groundwater by reducing leachate formation through municipal and industrial wastes Installation of the cover has greatly improved the aesthetics of the area and increased the value of the wildlife habitat The design concept is being considered as a model that will become national policy.

• The author also knows of many other phyto-cover applications in MA, OH, MD,

NC, MI, PA, NY, NJ, and IL.

In 1998, USEPA began an effort to establish a design database and improve ical prediction methods for alternative landfill covers The initial task of the AlternativeCover Assessment Project (ACAP) was to catalog past and existing research effortsinto measurement of cover performance and to describe the current state of numericalprediction methods The primary criterion was to measure percolation directly Severalresearch sites operated by branches of the federal government were included in thisstudy These sites include the national laboratories at Hanford, Sandia, Los Alamos,Savannah River, and Idaho Falls, and DOE locations at Monticello, UT, Nevada TestSite, DoD locations at California, Hawaii, Colorado, Utah, and others

numer-7.6.1 BeneÞts of Phyto-Covers over Traditional RCRA Caps

In addition to satisfying the critical antileaching requirement, phyto-covers vide a number of significant pollution control, ecological, and economic benefitswhen compared to traditional RCRA caps:

pro-• A phyto-cover actually enhances natural biodegradation processes, instead of interfering with them, as a RCRA cap would.

• A gas-permeable phyto-cover allows for passive venting of gaseous byproducts

of biodegradation and allows oxygen to move into the fill to facilitate additional biodegradation.

• A phyto-cover provides a forest ecosystem and an attractive alternative to an RCRA cap.

• A phyto-cover can be installed with less cost and less risk to public safety than

a RCRA cap and, once the cover is established, the system has a natural stability that minimizes long-term maintenance requirements.

7.6.2 Enhancing In Situ Biodegradation

Installing a phyto-cover at a site has the potential to enhance biodegradation ofwaste materials, including organic waste and contaminants, in the root zone Innatural ecosystems, high concentrations of indigenous soil microorganisms are found

in association with plant roots, because the roots exude a wide variety of compoundssuch as sugars, amino/acids, carbohydrates, and essential vitamins These com-pounds not only sustain the microbial consortia, which can degrade many organiccompounds directly, but also enhance and accelerate cometabolic degradation of

other pollutants resistant to direct degradation In addition, the plants themselveswill take up and metabolize or volatilize some of the organic contaminants in thefill Finally, exuded organic acids also help in sequestering and immobilizing anymetals present in the root zone By contrast, a RCRA cap provides no stimulation

to natural biodegradation and would be expected to substantially change geochemical conditions in the fill by trapping the gaseous byproducts of biodegra-dation (e.g., methane, carbon dioxide, and ammonia), thereby affecting factorscritical to natural attenuation mechanisms, such as pH and REDOX potential.The main reason for the enhanced in situ biodegradation in landfills with phyto-covers is the ability of the atmospheric oxygen to transfer into the landfill Theprimary mechanism transferring oxygen into the landfill is diffusion into the soilfrom the atmosphere, based on an excellent summary shown below:14

bio-The exchange of gases between the soil and the atmosphere … is facilitated by two mechanisms: mass flow and diffusion Mass flow of air, which is due to pressure differences between the atmosphere and the soil air, is less important than diffusion

in determining the total exchange that occurs It is enhanced, however, by fluctuations

in soil moisture content As water moves into the soil during a rain … air will be forced out Likewise, when soil water is lost by evaporation from the surface or is taken up by plants, air is drawn into the soil Mass flow is also modified slightly by other factors such as temperature, barometric pressure, and wind movement Most

of the gaseous interchange in soils occurs by diffusion.

The minimum rate of oxygen diffusion at the bottom of the root zone wasestimated to be 5 ¥ 10–8 grams per centimeter, squared per minute, or 2340 poundsper year per acre.14 The maximum rate could be up to 9200 pounds per year peracre Over the surface of a 30-acre landfill, this translates into at least 70,000 pounds

of oxygen per year into the landfill, which facilitates stabilization of the waste Bycontrast, the single-barrier cap would admit only an estimated 75 pounds of oxygen

or about one tenth of one percent of the influx that could support the aerobic naturalattenuation mechanisms (Figures 7.7a and b).15

Unlike RCRA caps, which are essentially impermeable to gases and thereforerequire elaborate gas venting systems to deal with gases and vapors generated bybiodegradation of the fill, a phyto-cover is porous and permeable to gas A phyto-cover can thus eliminate the need for a gas collection system at many sites Equallyimportant, a phyto-cover will allow oxygen to migrate into the fill, which will help

to support additional aerobic biodegradation and thereby hasten the completion ofthe waste life cycle

7.6.4 Ecological and Aesthetic Advantages

Both a phyto-cover and an RCRA cap are designed to be vegetated on the surface,but vegetation on a phyto-cover has the appearance of a tree farm and, eventually,

a forest, and serves the same ecological function as a forest while the RCRA cap iscovered with grass and, in order to protect the impermeable liner, must be preventedfrom functioning like local natural ecosystems Specifically, maintenance of theintegrity of the RCRA cap’s impermeable layer dictates that deep-rooted plantspecies, such as trees and shrubs, not be allowed to colonize the site through naturalsuccession Moreover, protection of the impermeable liner also requires that smallburrowing mammals, such as those normally associated with a meadow, must beperpetually monitored for and eliminated when found By contrast, the trees of aphyto-cover provide nest sites for birds and other arboreal species and readily acceptin-fill by shrubs and native tree species, as deemed appropriate under site manage-ment criteria Because no animal is likely to excavate below the deep root zone, it

is not necessary to prevent native fauna from inhabiting the phyto-cover Besidesoffering a preferred natural ambiance, the phyto-cover forest would also serve thecommunity as a noise pollution buffer and assist incrementally with global climateissues by fixing substantially more carbon dioxide from the atmosphere than a grassRCRA cap

7.6.5 Maintenance, Economic, and Public Safety Advantages

The ongoing maintenance requirements for an established phyto-cover are imal Although relatively intensive monitoring for disease and pests is needed duringthe three growing seasons that the trees need to become fully established, mainte-nance activities thereafter are expected to be minimal because of the self-healingnature of the phyto-cover Like a natural forest, the phyto-cover is expected to beresistant to wind and water erosion Unlike a RCRA cap, which can suffer cracks,rips, and tears due to factors such as differential settling or physical intrusion, thephyto-cover maintains its integrity and actually heals itself with new root growth inresponse to physical disturbances Thinning of trees may be undertaken in the future

min-to avoid crowding as the trees reach their mature size However, the trees cut in athinning operation represent a valuable forestry crop, so revenue from their saleshould compensate for the operation’s costs

The lower economic cost of the phyto-cover compared to the RCRA cap is panied by lower noneconomic social costs in the form of safety risks Studies haveshown that remedy implementation imposes risks of injury and death to site workers,neighbors, and the public using transportation routes These risks are more certain andtypically substantially greater in magnitude than risks to the public from exposure tosite contaminants For example, assuming that bulk construction materials can be found

accom-at an average distance of 15 miles (i.e., 30 miles round trip) and using the U.S truckfatality rate of 4.7 ¥ 10–8/mile, construction of RCRA “C” cap at a 30-acre landfillsite could lead to an estimate of transportation fatalities risk of 0.033.16 This estimatewill be further increased if the nontruck driver fatalities estimate is combined withthis Since phyto-covers require less site work and fewer truckloads of importedmaterial, such as borrow soil and gas collection layer sand, constructing a phyto-coverinstead of an RCRA cap would involve less risk of an accidental injury or fatality to

a construction worker and lower risks of traffic incidents associated with truckloads

of construction materials carried over local roads

Finally, unlike a RCRA cap, which locks the site into an “impermeable barrier”strategy, the phyto-cover system can be operated in a flexible way to take into accountthe results of ongoing performance monitoring data For example, if performancedata show that native species perform as well as hybrid poplars in preventinginfiltration, then the natural transition to native species can be accelerated, to enhancethe ecological service provided by the area By the same token, in the unlikely eventthat performance data show that a part of the cover is not performing to expectation,then a supplementary measure such as additional “sponge” or denser planting would

be available to improve performance

7.7 PHYTO-COVER DESIGN

The typical components of an engineered phyto-cover system consist of tative cover soils (existing and supplementary), soil amendments, nonsoil amend-ments, understory grasses and plants, and trees An irrigation system is an optionalcomponent to ensure sufficient water for tree growth in case of drought Irrigated

vege-trees grow more rapidly, thus meeting closure objectives in less time; however, there

is often lack of a convenient water source or on-site operation and maintenancepersonnel to make an irrigation system feasible at a site The need for on-siteirrigation should be based upon the expected water consumption of the trees

7.7.1 Vegetative Cover Soils

The existing cover soil at many sites is sufficient to support an adequate rootsystem for healthy tree growth This is evidenced by the vigorous growth of treesoften seen at abandoned landfills (Figure 7.8); however, the ability to grow trees isnot evidence that percolation and leachate production are controlled Typically,natural stands of vegetation are not effective at controlling percolation Therefore,sufficient soil and nonsoil amendments may need to be added to meet the require-ments for tree growth, and to achieve minimum land surface slopes to promotesurface drainage and to provide sufficient soil water holding capacity for storage tofunction as an adequate “sponge.” The amount of soil and nonsoil amendmentsnecessary must be determined by site-specific information, often collected in thelater stages of design

Any supplemental cover soil added to achieve the required grades, as well assufficient water storage capacity, will comprise common borrow soils Supplementalsoil should be placed in 6-inch thick and loose lifts, and be lightly compacted toachieve the minimum slope and thickness This material is typically available fromseveral sources in the vicinity of most sites; the specific local source usually dependsupon availability during the construction period The surficial lift of supplementalsoil and existing cover, depending upon which is exposed at the final grade surface,

must be ripped in two directions following final grading to assure noncompactionand to prepare the surface to receive the nonsoil amendments.

The addition of nonsoil amendments will increase the water-holding capacityand nutrient transfer properties of the common borrow soils Typical nonsoil amend-ments include compost, chipped wood, digested sewage biosolids, lime-stabilizedsludge, manure, and other organic biomass The incorporation of this type of organicmatter into the existing and supplemented soils will greatly increase the tilth, fertility,and water-holding capacity of the soil, and further reduce percolation through thecover Biosolids compost and lime-stabilized sludge are readily available through acompost contractor Typically a minimum 6-inch thick layer of organic amendmentsneeds to be applied to the soil surface after achieving final grade This material isspread evenly in a six-inch thick layer on the area to be planted with the engineeredphyto-cover system and is ripped into the surficial soils to a depth of 14 to 18 inches.Ripping is performed in both an east-west and north-south orientation in order toachieve a uniform mixing within the soil profile Finally, the site is tilled in prepa-ration for planting

If the organic materials used for the nonsoil amendment have a high carbon tonitrogen ratio, fertilizer is added along with organic amendments to aid in stabilizingthese amendments and to provide sufficient nutrients to the rooting plants Thisorganic amendment is expected to supply all micronutrients required by the plants

A mineral fertilizer is also added as needed, based on nutrient analyses of the appliedcompost, to supplement the macronutrient reserves of nitrogen, phosphorous, andpotassium All amendment addition, ripping, and tilling is completed prior to under-story planting in the fall and before the trees are installed in the followingearly spring

7.7.3 Plants and Trees

The area to be planted will generally exhibit a minimum 2% or greater grade;therefore, stabilization of the site cover material remains necessary to prevent ero-sion Understory planting will be established for early erosion control and wateruptake during the first year Understory establishment is a combination of annualand perennial grasses, such as varieties of rye, oats, wheat, barley, and fescue, applied

at a rate of 20 to 40 pounds per acre This mixture of seed is designed to meet theshort- and long-term objectives of the understory Annual species will be fast growing

to control near-term erosion; perennial grasses will be deep-rooted species selected

as the primary long-term understory for the site The long-term effectiveness of theoverstory is dependent upon establishment and long-term maintenance of the under-story, which understory depletes shallow soil moisture, turn causing tree roots togrow deeper to meet water requirements As discussed earlier, the success of a phyto-cover is dependent upon establishment of deep-rooted trees to create a sufficientsponge to store soil moisture in the dormant season

The trees normally selected for construction of a phyto-cover are hybrid poplars

of the variety Deltoides x nigra.10,13 The candidate varieties, DN-21, DN-34, OP 367and others, are planted based on demonstrated growth ability and hardiness in theenvironment The poplars are installed as either rooted plants or whips at a density

of approximately 1200 trees per acre.15 The rows are located by measurement andflagged, and the trees installed by a tractor and mechanical planter These trees aretypically planted with an in-row spacing of 3 feet and a row spacing of 10 to 13feet They are planted in rows positioned along the land elevation contours, perpen-dicular to slopes to aid in reducing sheet flow velocities and surface erosion

7.8 COVER SYSTEM PERFORMANCE

The engineered phyto-cover system should be designed to meet the post-closureand remediation objectives established for any landfill site as specified below:

• Minimize infiltration of precipitation through the cover system into the waste to protect groundwater quality at the site.

• Resist surface soil erosion by wind and precipitation.

• Minimize long-term maintenance.

• Protect human health and the environment.

• Offer post-closure and future beneficial use.

The achievement of these objectives is outlined in this and subsequent sections

7.8.1 Hydrologic Water Balance

The engineered phyto-cover system is designed to mature into a remedial systemthat exceeds the hydrologic performance of more conventional cover systems How-ever, instead of acting as a constructed barrier layer, which diverts precipitation fromthe cover area as surface runoff or internal drainage, this system intercepts and usesthe water for plant growth In other words, the engineered phyto-cover functions as

a sponge and pump system, with the root zone acting as the sponge, and trees acting

as the solar-driven pumps In contrast to restrictive permeability barrier design, theengineered phyto-cover design involves the storage of free water in soil pores andthe extraction of stored water by the tree roots

The effectiveness of engineered phyto-cover systems as landfill closure systemshas been demonstrated at sites in the U.S At sites in various climates with engineeredand agronomically optimized growing conditions, rapidly growing poplar trees arecapable of transpiring all natural precipitation that infiltrates into a site While theperformance of engineered phyto-cover systems has been demonstrated, a proventool to design phyto-covers is not available Therefore, to support the design anddemonstrate the effective performance of phyto-cover systems, this section discussessome fundamental scientific methods of water balance analysis

As discussed previously, the phyto-cover system utilizes specially selected treeswith a grass understory to optimize evapotranspiration and achieve the equivalent

performance of a conventional barrier cover system This alternative landfill coversystem has been designed to minimize percolation to the waste by incorporating alandfill soil cover with sufficient evapotranspirative and water holding capacity tostore precipitation temporarily in the nongrowing season for subsequent evapotrans-piration by vegetation in the growing season The two key design elements inengineering a phyto-cover system are 1) determining the thickness and materialcomposition of the soil cover system required to provide sufficient water storagecapacity; and 2) incorporating a supportive phyto-cover system to access water stored

in the soil cover system for evapotranspiration to the atmosphere

Moisture flow and moisture content in a landfill are extremely important to thedynamic processes of decomposition and potential leachate generation The funda-mental means to assess the moisture conditions is through an evaluation of variousprocesses comprising a water mass balance A water mass balance analysis is an

“accounting” procedure for tracking the moisture inputs to storage and the moistureoutputs that influence the potential flux of water through the cover into the waste.The primary elements of a water mass balance include precipitation, surface runoff(R/O), potential evapotranspiration (PET), infiltration (I), soil moisture storage (ST),actual evapotranspiration (AET), and flux (or percolation) of water through thesystem The water shedding efficiency of a cap is then derived by calculating thepercentage of flux relative to total precipitation The phyto-cover system designconcept involves maximizing efficiency by optimizing ET, runoff, and soil moisturestorage to minimize infiltration, flux, and potential leachate generation The waterbalance accounting for a phyto-cover can be summarized by the following equationand Figure 7.9:

Percolation = Precipitation – Runoff – Evapotranspiration – Moisture Storage

(7.3)

The water mass balance processes within a landfill are typically evaluated usingthe hydrologic evaluation of landfill performance (HELP) model, developed by theWaterways Experiment Station.17 The applicability of this model to design andevaluation of an engineered phyto-cover system has been reviewed, and it has beendetermined that the HELP model is inappropriate for this analysis because of severalcomputational deficiencies.18,19 The HELP model was developed based on assump-tions pertaining to water management through low permeability soil covers withvegetative covers comprising short-rooted grasses No opportunity exists for userinput of higher ET values more representative of plant species with significantlyhigher potential water uptake than the short grasses assumed by the HELP model.Therefore, the model significantly underestimates evapotranspiration from trees andother deeply rooted vegetation that are key elements of a phyto-cover system Thisapplication limitation of the HELP model results in an overestimation of infiltrationand coincident underestimation of efficiency (overestimation of drainage) if themodel were to be applied to an evaluation of a phyto-cover system

A detailed assessment of various computer models used for landfill cover designsduring the early phases of the alternative cover assessment program (ACAP) came

to similar conclusions.11,12 Of the four codes tested, HELP was the most widely used

for landfill design, and the most user-friendly and highly dependable HELP tions consistently provided the highest estimates of drainage Three concerns withHELP were 1) a nonrealistic response of increased drainage as available watercapacity increased, 2) insensitivity of drainage to thickness of the cover surfacelayer, and 3) consistent overprediction of drainage EPIC was also relatively easy

predic-to use, but consistently underpredicted drainage in comparison predic-to other codes Thestudy suggested that Richards’ equation-based codes (HYDRUS–2D, UNSAT–H)were better able to capture the behavior of alternative landfill covers than simplewater balance codes such as HELP and EPIC

Although the HELP model itself cannot accurately simulate the hydraulic effects

of an engineered phyto-cover system, the water balance method that is the mental principle applied within the HELP model has been employed to evaluate theperformance of vegetative cover systems.20 These same scientific principles areemployed to design and evaluate the performance of an engineered phyto-coversystem with a new software tool called PHYTOSOLV.15,21 In using the water balancemethod, the first step is to acquire accurate precipitation records applicable to thesite and encompassing various extreme wet and dry periods The second step is todetermine the quantity of surface water runoff and infiltration (which are functions

Depth of Capillary Zone Draw

Root Depth

Storage Infiltration

Soil Evaporation

Surface Evaporation Leaf Transpiration

Surface Evaporation Surface Cover

Interception

Canopy Interception Precipitation

Potential Infiltration

1) Surface Litter or

Compost

2) Soil and Nonsoil

Amendments

3) Waste Rootable Upper

Layer (Contributes toward

Storage)

Thickness of the Arrow is

Proportional to the Volume of Water

of the site soils, slope, and surface texture) Infiltration is computed as the difference

between precipitation rates for the site and surface-water runoff from the soil cover

The third step is to apply PHYTOSOLV, assuming a variety of soil cover depths, to

generate a range of annual hydrologic water balances using daily precipitation data

Finally, a supporting phyto-cover system is designed that would access infiltrated

soil water throughout the entire extent of root growth (the “sponge”), and the

necessary evapotranspiration rate (the “pump”) required to deplete soil moisture

during the growing season is computed This iterative water balance analysis is used

to select the appropriate soil cover design to best achieve desired hydraulic

perfor-mance, thereby minimizing generation of leachate The measure of performance

for the designed phyto-cover is compared to the water-shedding efficiency of

tradi-tional barrier cover systems Presented below is a discussion of each of these steps

and the basis for the general engineered phyto-cover system design

7.8.2 Precipitation

Long-term precipitation data need to be assembled from the closest weather

station to evaluate local hydrologic conditions There are no established regulatorily

approved procedures or protocols to evaluate the hydrologic performance of a

phyto-cover design Therefore, term data are needed in order to characterize the

long-term precipitation trends and extremes Typically, precipitation can vary widely from

site to site for a given year, season, or month To demonstrate this variability, data

should be assembled summarizing average monthly and annual precipitation for

decades at weather stations near any given site For example, during a long period

at a site in Maryland, the average annual precipitation varied from a minimum of

26.29 inches in 1965 to a maximum of 62.36 inches in 1996 Similar variability can

also be observed in monthly precipitation totals To the extent practical, these

dynamics must be accounted for in the design of the phyto-cover system to

demon-strate adequate performance under extreme weather conditions The application of

these data to evaluate the phyto-cover design assumes that daily precipitation totals

are the result of individual storm events

Runoff from the designed phyto-cover is computed using the USDA Soil

Con-servation Service (SCS) curve number model.22 The model computes direct runoff

from an individual storm event as a portion of total precipitation (Figure 7.10) The

method was developed from field studies by measuring runoff from various soil

cover, land slope, and soil type combinations Curve numbers were developed based

upon each of the combined hydrologic effects of these factors and enable the model

to be applied to any area within the U.S The curve number model is widely used

and is incorporated into the HELP model and other agronomic models to compute

rainfall runoff and other elements comprising a water balance The major deficiency

in this model is that it underestimates runoff from small precipitation events This

discrepancy results in overestimates of infiltration and the amount of water that must

be managed by the cover system.3 Consequently, the resultant engineered

phyto-cover is overdesigned and conservative: the engineered phyto-cover has the

ability to control more infiltration than it is designed to manage.15

(7.4)

where

Q = runoff (in)

P = precipitation (in)

S = potential maximum retention after runoff begins (in)

Ia = initial abstraction (in)

The initial abstraction is all water loss before runoff begins It includes water

detained in surface depressions, as well as water intercepted by vegetation, evaporation,

and infiltration The initial abstraction is highly variable but from data collected from

small agricultural watersheds, Ia was approximated using the following equation:

By eliminating Ia as an independent parameter, this approximation allows use of

a combination of retention storage (S) and precipitation (P) to predict a unique runoff

amount Substituting Equation 7.5 into Equation 7.4 gives

(7.6)

Curves on this sheet are for

the case Is = 0.2S, so that

60 50

where S is related to the soil and cover conditions of the watershed through the

curve number CN CN has a range from 30 to 100, and is related to S by the following

equation:

(7.7)

The use of the SCS runoff equation for this analysis assumes that the difference

between precipitation and runoff is infiltration.15

The curve number can be estimated by either using the HELP model or other

computations The HELP model computes a curve number based upon final grade,

soil type, and vegetative cover Using this model is recommended because it

objec-tively estimates a curve number based upon the final design In addition, the methods

in the HELP model were developed and approved by the USEPA When using the

model, the minimum final grade should be specified for the land surface slope and

a good vegetative cover be assumed for the understory These two assumptions

ensure that the selected curve number is conservative (minimize runoff/maximize

infiltration)

Potential evapotranspiration (PET) is a measure of the maximum rate at which

evapotranspiration can occur when adequate soil moisture is available for utilization

by vegetation These data are measured in the field utilizing lysimeters planted with

single species covers (usually perennial grasses) Soil moisture levels are maintained

at optimum levels and evapotranspiration is measured by weighing the lysimeter

Data collected through these methods are the most reliable and most defendable

estimates of potential evapotranspiration; however, measured site-specific data are

not readily available for most sites

The monthly potential evapotranspiration rates measured for grasses are adjusted

to best represent the supplemental evapotranspiration available from the trees This

step is performed by incorporating a consumptive-use coefficient (Kc) applicable to

the trees utilized in the phyto-cover design A consumptive-use coefficient of 1.35

was measured for areas with cottonwood trees, willows, and grass.15,23 This value is

near the low end of the range for consumptive-use coefficient values derived for

densely planted trees in other parts of the U.S.; it has been reported that

consumptive-use coefficient values for densely planted trees vary from 1.3 to 1.6.24 The

cotton-wood is one species used to develop the hybrid poplar trees selected for the

phyto-cover system Hybrid poplar trees have been developed specifically to achieve a high

consumptive use coefficient, in addition to disease resistance and high growth rates;

therefore, the selection of a consumptive-use coefficient of 1.35 is conservative for

this engineered phyto-cover system

The consumptive-use coefficient is used to adjust the measured potential

evapo-transpiration for grasses only during the growing season for trees — April through

October (Figures 7.11a and b) The growing season begins with approximately 10%

SCN

=1000-10